Engagement milling

ABSTRACT

A method using a computer for generating a spiral-like tool path for milling a region of a workpiece is disclosed. The method includes the steps of: creating a family of concentric indexed circular arcs at each of two or more separate and distinct selected points within the region; determining parameters of a first set of blends to connect together the circular arcs of adjacent families of the circular arcs having an identical index to form a plurality of isoloops; determining parameters of a second set of blends for blending between adjacent isoloops to form the spiral-like tool path, and generating instructions for controlling the milling cutter in accordance with the generated tool path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/129,293, filed May 29, 2008, which is a divisional application ofU.S. patent application Ser. No. 11/112,396 entitled “EngagementMilling”, filed Apr. 22, 2005, now U.S. Pat. No. 7,577,490, issued Aug.18, 2009, which is a continuation-in-part of U.S. patent applicationSer. No. 11/070,430, filed Mar. 2, 2005, now U.S. Pat. No. 7,451,013,which claims the benefit of U.S. Provisional Application No. 60/566,586,filed Apr. 29, 2004, which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to computer aided manufacturing and morespecifically to a method and apparatus for generating a computernumerical control program for controlling a numerical control machine.

In milling a workpiece using a numerical control (NC) machine, it isdesirable to remove material from the workpiece as fast as possibleconsistent with long tool life. Controlling tool load throughout eachtool path to be within a preferred range is of primary importance forachieving long tool life. Therefore, a computer aided manufacturing(CAM) system that creates a computer numerical control (CNC) programhaving tool paths which maintain the tool load within the preferredrange throughout each tool path, while generating tool paths whichcumulatively remove the greatest amount of material in the shortestpossible time, is desirable for cost effective milling.

It is widely accepted that maintaining a constant engagement of themilling cutter over the tool path is a major factor in controlling toolload. However, existing machining strategies, such as the widely useddirection parallel (zig or zig-zag) and contour machining strategies,and their variations, generate tool paths which are dictated by a partboundary, a material boundary or a combination of the part boundary andthe material boundary, and not by considerations of maintaining theengagement of the milling cutter constant over the tool path. Suchmilling strategies force the milling cutter to execute sharp turnsresulting in widely varying cutter engagement. Such variations in toolengagement cause spikes in tool load producing undesirable effects suchas shorter tool life, chatter and even tool breakage.

In order to mitigate the effect of spikes in the tool load caused by asudden increase or decrease in tool engagement, users of existing CAMprograms typically reduce the feed rate for an entire tool path to beconsistent with the peaks of the spike loads. Such reductions in feedrate can result in significant reductions in milling efficiency.

In order to resolve problems arising from existing machining strategies,methods have been proposed in which the machining conditions such asengagement angle and feed rate of a tool path are evaluated after thetool path is created. The tool path is then modified according tovarious criteria. Such modifications include adding additional segmentsto portions of the tool path or reducing the curvature of the tool pathin those local portions of the tool path in which the tool engagement isexcessive. However, by piecemeal correction of local machiningconditions in the tool path, machining conditions in other portions ofthe tool path may be altered in unpredictable ways. Further, while theaforementioned types of tool path modifications avoid large spikes inthe tool load, these types of modifications typically result inadditional length to the total tool path and do little to maintain arelatively constant engagement of the milling cutter over the entiretyof the tool path.

A further problem with existing CAM programs is reliance on simplemeasures (i.e. chip load) as a measure of the load on the millingcutter. Real tool load is a function of both undeformed chip thickness(UCT) and tool engagement. In order to maintain the tool load within apreferred range, the feed rate of the milling cutter should be based onboth the UCT and the tool engagement.

In consideration of the above, it would be desirable for a CAM system tocreate, by a direct process, a CNC program which generates tool pathsfor milling a region of a workpiece which are based on controlling thetool engagement and for which the tool engagement is maintained to besubstantially constant over a major portion of the tool path withoutexceeding a maximum value of the tool engagement. Further, it would bedesirable for the CAM system to create, by a direct process, tool pathssuitable for milling all types of convex and concave geometries.Further, it would be desirable for the CNC program to provide forcontrolling the feed rate of the tool such that the load on the tool isautomatically maintained within a preferred range over the entirety ofthe tool path.

BRIEF SUMMARY OF THE INVENTION

Briefly stated the invention is a method for generating a tool path formachining a pocket with a milling cutter where the tool path includes afirst portion, a second portion and a transition portion connectingtogether the first portion and the second portion. The method forgenerating a path for the transition portion comprises the steps of:determining a radius of a first arc based on a stepover of the millingcutter; determining a radius of a second arc based on the radius of thefirst arc and a maximum engagement of the milling cutter; and situatingthe first arc so as to connect the first portion to the second portionin a tangent continuous manner; situating a third arc so as to betangent continuous with a first end of the first arc at a first end ofthe third arc and intersecting a second end of the first arc at a secondend of the third arc; and situating a fourth arc, so as to be: (1)tangent continuous with the second end of the third arc, (2) tangent tothe second arc and (3) tangent continuous with either the first portionor the second portion.

In another aspect the invention the invention is an article ofmanufacture comprising a computer readable medium having stored thereoncomputer executable instructions for generating a tool path formachining a pocket with a milling cutter, the tool path including afirst portion, a second portion and a transition portion connectingtogether the first portion and the second portion The instructions forgenerating a path for the transition portion comprise the steps of:determining a radius of a first arc based on a stepover of the millingcutter; determining a radius of a second arc based on the radius of thefirst arc and a maximum engagement of the milling cutter; and situatingthe first arc so as to connect the first portion to the second portionin a tangent continuous manner; situating a third arc so as to betangent continuous with a first end of the first arc at a first end ofthe third arc and intersecting a second end of the first arc at a secondend of the third arc; and situating a fourth arc, so as to be: (1)tangent continuous with the second end of the third arc, (2) tangent tothe second arc and (3) tangent continuous with either the first portionor the second portion.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofpreferred embodiments of the invention, will be better understood whenread in conjunction with the appended drawings. For the purpose ofillustrating the invention, there is shown in the drawings embodimentswhich are presently preferred. It should be understood, however, thatthe invention is not limited to the precise arrangements andinstrumentalities shown.

In the drawings:

FIG. 1 is a functional block diagram of a computer aided manufacturingsystem according to the present invention;

FIG. 2 a is an illustration of a tool engagement for a straight path fora given radial depth of cut;

FIG. 2 b is an illustration of the tool engagement for a convex path forthe given radial depth of cut;

FIG. 2 c is an illustration of the tool engagement for a concave pathfor the given radial depth of cut;

FIG. 3 is a functional flow diagram of a preferred method forcontrolling the feed rate of a milling cutter;

FIG. 4. is a diagram of showing the construction of a two arc filletaccording to a first preferred embodiment;

FIG. 5 is functional flow diagram of a preferred process for determininga predetermined value of engagement according to the first preferredembodiment;

FIG. 6 is an illustration of a region in a workpiece having an arbitrarymaterial or part boundary;

FIG. 7 is an illustration of typical case where material has been leftremaining in the corners of a rectangular pocket by previous passes of atool;

FIG. 8 a is an illustration showing that the location of the maximumengagement of the milling cutter is where the leading edge of themilling cutter reaches the midpoint of the pass;

FIG. 8 b is an illustration showing that the engagement is smaller oneither side of the maximum engagement point;

FIG. 9 is an illustration defining the parameters for calculating theradius and stepover of the next in-process material boundary accordingto a second preferred embodiment;

FIG. 10 is an illustration defining the parameters for calculating thestepover for a slot according to the second preferred embodiment;

FIG. 11 is an illustration of the steps of an iterative search accordingto the second preferred embodiment;

FIG. 12 is another illustration of the steps of an iterative searchaccording to the second preferred embodiment;

FIG. 13 is an illustration of the tool path for milling a converging anddiverging region according to the second preferred embodiment;

FIG. 14 is an illustration of a workpiece in which a step, open on twosides, is to be milled;

FIG. 15 is an illustration of a tool path proceeding in the workpieceuntil a predetermined engagement is reached;

FIG. 16 is an illustration of the in-process material boundary requiredfor milling the final cut with a substantially constant engagement;

FIG. 17 illustrates a graphical construction of the in-process materialboundary according to a fifth preferred embodiment;

FIG. 18 illustrates an alternate method for constructing the in-processmaterial boundary according to the fifth embodiment;

FIG. 19 illustrates another alternate method for constructing thein-process material boundary according to the fifth embodiment;

FIG. 20 illustrates the creation of a medial axis transform for atrapezoid according to a sixth embodiment;

FIG. 21 illustrates the creation of the outer circles of two circlefamilies according to the sixth embodiment;

FIG. 22 illustrates forming concentric circles and isoloops according tothe sixth embodiment

FIG. 23 illustrates the in-process material boundaries formed from aspiral-like tool path according to the sixth embodiment;

FIG. 24 illustrates a family of concentric circles according to a firstembodiment;

FIG. 25 illustrates arcs tangent to the concentric circles according tothe first embodiment;

FIG. 26 illustrates a tool path joining the arcs together according tothe first embodiment.

FIG. 27 illustrates constructing normals along an in-process materialboundary;

FIG. 28 illustrates constructing an engagement chord;

FIG. 29 illustrates creation of a tool center point;

FIG. 30 illustrates fitting a spline to the series of points depicting afirst trial boundary;

FIG. 31 illustrates the geometry for determining the parameters of thetool path in accordance with the seventh preferred embodiment;

FIG. 32 a illustrates the in-process material boundaries formed bygenerating the tool path with an outside-in process; and

FIG. 32 b illustrates the in-process material boundaries formed bygenerating the tool path with an inside-out process.

FIG. 33 illustrates the method of determining the engagement of amilling cutter milling a circular pass in a slot;

FIG. 34 illustrates the change of engagement over a circular tool path;

FIG. 35 illustrates the in-process material boundaries formed in a slotin accordance with the eighth embodiment;

FIG. 36 is a flowchart of a process for determining the maximum value ofpitch in accordance with the eighth embodiment;

FIG. 37 illustrates the in-process material boundaries formed in anacute corner in accordance with the eighth embodiment;

FIG. 38 illustrates an in-process material boundary corresponding to thetool path generated in accordance with the ninth embodiment;

FIG. 39 shows in-process material boundary corresponding to thetransition portions of the in-process material boundary shown in FIG.38;

FIG. 40 shows the path taken by a milling cutter when milling thein-process material boundary corresponding to the transition portion ofthe tool path shown in FIG. 38;

FIG. 41 shows a tool path for enlarging a region in accordance with thetenth embodiment; and

FIG. 42 shows a geometric construction for determining a radius for eachpass of the tool path of the tenth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

The following definitions are to be applied to terminology used in theapplication:

Arc—a curved line—not necessarily circular.

Axial Depth of Cut—the depth of cut in the axial direction of a millingcutter.

CAM—computer aided manufacturing.

CAM program—a computer program used for generating a computer numericalcontrol (CNC) program for milling a workpiece by a numerical control(NC) machine.

center-line feed rate—the feed rate measured at the rotational axis ofthe milling cutter.

Chip Load—the distance a tool moves as a single flute of the tool cutsmaterial=feed rate/(spindle speed×number of flutes)

Circular arc—the set of all points equidistant from a fixed point calledthe center, i.e. a portion of a circle.

CNC program—the output of a CAM program consisting of a set ofinstructions, i.e. a program, defining machining conditions andmovements of a tool mounted in a numerical control machine relative to aworkpiece mounted in the numerical control machine.

Feed rate—the rate at which a milling cutter moves along a tool path,measured at the rotational axis of the milling cutter.

In-process material boundary—a boundary of the workpiece established bya cutting operation. The in-process material boundary is always offsetfrom the tool path by the radius of the milling cutter.

Material boundary—a part boundary or in-process material boundary.

Milling cutter—a cutter which rotates about a rotational axis including,but not limited to, end mills, face mills, shell mills, slab mills,plunge mills, single angle cutters, dovetail cutters, keyseat cutters,T-slot cutters, concave and convex cutters and gear hob cutters.

Part boundary—the fixed boundary of the workpiece to be attained at thecompletion of the milling operation, generally defined by a blue printof the part or by a computer aided design (CAD) program.

Pass—a portion of the tool path for which the milling cutter is incontinuous contact with the workpiece.

Peripheral (effective) feed rate—the rate at which a milling cuttermoves along a tool path, measured at the periphery of the millingcutter.

Programmed feed rate—a reference value of the feed rate, typicallyselected by the NC programmer.

Radial depth of cut—the amount of material removed in the radialdirection of the milling cutter.

Smooth curve—a curve in which the unit tangent to the curve iscontinuous at every point along the curve.

Spindle speed—the rotational speed of the tool.

Stepover (or stepover value)—the distance normal to a tool path betweena first pass and a substantially parallel second pass.

Undeformed chip thickness—the maximum thickness of a chip removed by asingle cut of a single flute of a milling cutter.

Tangent continuous—forming a smooth curve.

TEC—tool engagement curve—a tool path which causes a milling cuttertraversing the tool path to have a substantially constant engagementangle.

TEM—tool engagement milling.

Tool engagement—The region of contact between the tool and the workpieceat the in-process material boundary, frequently expressed as an angle.

Tool path—the path of the rotational axis of a single milling cutterwhen milling a workpiece.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, wherein like numerals are used to indicatelike elements throughout the several figures and the use of theindefinite article “a” may indicate a quantity of one, or more than oneof an element, there is shown in FIG. 1, a functional block diagram of apreferred embodiment of a computer aided manufacturing (CAM) system 10for creating a numerical control (CNC) program which, when provided to anumerical control (NC) machine, enables the NC machine to produce aphysical object having a shape conforming to a specified part boundary.

Preferably, the CAM system 10 is a standalone programmable computerplatform having an open type of computer architecture of a kind commonlycalled a personal computer. Preferably, the CAM system 10 transfers theCNC program to a controller of an NC machine using one of any well knownwire or wireless interface standards. Alternatively, the CNC program maybe recorded on a removable media such as a floppy disk, a CD disk, amagnetic tape or a paper tape, for transfer to the NC machine.Preferably, the computer employs a Pentium IV™ microprocessor chipmanufactured by Intel Corporation; a random access memory; non-volatilememory such as semiconductor read only memory, a hard disk, removableread/write memory drives such as a floppy disk drive and/or CD diskdrive, a paper tape and/or a magnetic tape drive; a keyboard; a mouse;and a video display. Preferably, the CAM system 10 utilizes the Windows™software operating system manufactured by Microsoft Corporation.However, the CAM system 10 is not limited to the aforementioned hardwareand software environment. It would be clear to those skilled in the art,other types of computers and operating systems, such as thosemanufactured by Apple, Inc., or International Business machines could beused within the spirit and scope of the invention. Alternatively, theCAM system 10 could be integrated within another computer system such asthe computer system of the NC machine.

Preferably, the CAM system 10 accepts information about the object to bemachined and the workpiece from which the object is to be machined, froma computer aided design (CAD) computer program. Such a CAD computerprogram accepts dimensional information about the object to be formedand the workpiece from which the object is to be formed, and translatesthe dimensional information into models defining the object and theworkpiece. Such CAD programs are well known to those in the art.Alternatively information about the object and the workpiece may beentered directly into the CAM system 10 by other known means, such as atablet and/or by manual key entry through a user interface.

The CAD system 10 includes a computer aided manufacturing (CAM) computerprogram 18. Preferably, the CAM computer program 18 accepts the modeloutput of the CAD program 18 for defining the parts boundary of theobject to be machined and the boundaries of the workpiece. The CAMcomputer program 18 is also capable of accepting information about theproperties of the material to be machined and of the milling cutters(also referred to as tools) for milling the object from the workpiece.Additional parameters associated with the milling operation such as theselection of a tool for each tool path, the area to be machined, and theprogrammed radial depth of cut and axial depth of cut to be used foreach tool path may be entered by a user, i.e. NC programmer. Preferably,the user provided information is entered from a keyboard or by a mousewith the aid of on-screen dialog boxes displayed on a video display.Alternatively, some or all of the aforementioned parameters may beprovided by the CAM computer program 18 itself.

The output of the CAM computer program 18 is a CNC program comprising aseries of instructions which provide for positioning of the workpiece,tool paths, feed rates etc. by the NC machine, as further describedbelow. Preferably, the language of the instructions generated by the CAMcomputer program 18 are compatible with the NC machine. The CAM computerprogram 18 may include a conversion routine for providing theinstructions for specific NC machines. Alternatively, the conversionroutine may be separate from the CAM computer program 18.

The CAM computer program 18 can be written in any suitable programminglanguage, including (but not limited to) C, C++ and Java, and can bedeveloped using standard programming practices to carry out the stepsand techniques described herein.

Engagement, refers to the portion of the periphery of a rotating millingcutter that is in contact, at any given moment, with the workpiece. Theengagement can be measured or expressed as an engagement angle, anengagement arc, or an engagement chord. The term engagement, is usedhereafter as representing all of the foregoing.

The engagement angle for cutting a straight tool path is given by Eq. 1as:

$\begin{matrix}{E = {\cos^{- 1}\left\{ \frac{r - S}{r} \right\}\mspace{14mu}{where}\text{:}}} & (1)\end{matrix}$

E=engagement angle

S=radial depth of cut, and

r=radius of the milling cutter.

The engagement angle changes when a tool path changes from a straightpath to a curve path. FIG. 2 illustrates, by example, the effect on theengagement angle when the tool path changes from a straight line to aconvex curve and to a concave curve using a tool with 0.25 inch radiusand a fixed stepover (radial depth of cut) of 0.2 inch. As can bereadily seen, compared with the engagement angle for cutting thestraight line, the engagement angle decreases when cutting the convexarc and increases when cutting the concave arc. Consequently, if aspecific radial depth of cut is selected for a straight tool path, theengagement angle will be different for curved tool paths.

The engagement angle for cutting a curved tool path with a radius ofcurvature R is given by Eq. 2 as:

$\begin{matrix}{E = {\pi - {\cos^{- 1}\left\{ \frac{{2r^{2}} + {2{R\left( {S - r} \right)}} - S^{2}}{2{r\left( {R - r} \right)}} \right\}}}} & (2)\end{matrix}$

Referring now to FIG. 3, there is shown a process 100, for generating aCNC program by the CAM program 18, for milling a workpiece by an NCmachine. Generally, the task of milling a workpiece is divided into aset of tool paths, where for each tool path a single milling cutter isused for milling a region of the workpiece. Each tool path comprises aseries of passes, where for each pass the single milling cutter is incontinuous contact with the workpiece. Associated with each tool pathare machining conditions such as feed rate and spindle speed to be usedby the same milling cutter for milling the region of the workpiece. Eachtool path may be automatically determined by the CAM program 18 ordetermined by the NC programmer based on his experience, knowing thecharacteristics of the CAM program 18, the characteristics of thematerial, the shapes to be machined and the tolerances of the finishedpart. Based upon the characteristics of the region to be machined, theCAM program 18 determines a starting point (coordinate) and a stoppingpoint for the tool path (step 100.2). Generally, unless the entiremilling operation is limited to a simple shape, such as a circularpocket, more than one tool path may be required for rough milling of theworkpiece. If a fine finish is required, at least one additional toolpath may be required for finish milling.

Generally, a specific milling cutter (step 100.4) for a each tool pathis selected based upon the NC programmer's experience and/or withguidance from the CAM program 18. Alternatively, the milling cutter maybe automatically selected by the CAM program 18 based upon parameters ofthe tool path, the type of material, whether the tool path is a roughingcut or a finish cut etc. Of particular importance to the efficiency ofthe tool path in milling the workpiece is the radius and the number offlutes of the selected milling cutter, since from these parameters andthe characteristics (e.g. hardness) of the material, a programmed feedrate is generally determined.

In each one of the preferred embodiments, a maximum engagement ispredetermined for each tool path based on the characteristics of theselected milling cutter, whether the type of milling is climb milling orconventional milling and the type of material to be milled (step 100.6).Preferably, the maximum engagement is automatically determined andstored by the CAM program 18. Alternatively, the maximum engagement maybe determined by the NC programmer. The input for determining themaximum engagement may also be a value of stepover, from which theengagement can be computed. Generally, for rough milling, the radialdepth of cut is selected to remove the maximum amount of material fromthe workpiece for each pass of the milling cutter consistent withabsence of chatter, vibration and excessive cutting force applied to theflutes of the milling cutter.

At step 100.7, a tool path is generated which expressly invokes criteriafor controlling the engagement of the milling cutter over the entiretyof a tool path. Such a process generates a tool path directly from themodel information defining the object to be machined and informationabout the workpiece, without first generating an initial tool pathwithout regard to the engagement of the milling cutter, and without theneed to subsequently modify the tool path to correct for deficiencies inthe tool path which would cause, for instance, rapid changes in toolload. Preferably, a single tool path is created for a region of theworkpiece. Preferably, the tool path is generated by a direct process,where the tool path consists of one or more passes, and where the methodcomprises defining a maximum engagement of the milling cutter anddefining each one of the one or more passes such that a value of theengagement, when traversing each one of the one or more passes, does notexceed the maximum value of engagement as described below.

Conventionally, the programmed feed rate of the milling cutter is basedon the chip load and spindle speed recommended by the manufacturer ofthe milling cutter, as modified by the experience of the NC programmer.However, as discussed above, when the engagement angle is other thanninety degrees, and/or the tool path is curved, the tool load is moreaccurately represented by the undeformed chip thickness (UCT) and thevalue of the engagement angle of the milling cutter.

Accordingly, at step 100.8, in each of the preferred embodiments, eithera criteria for controlling tool load or a criteria for controlling theperipheral feed rate of the milling cutter is selected for determiningthe feed rate of the milling cutter at points along the tool path. Inthe preferred embodiments, each tool path is divided into elements witheach element of the tool path having a different curvature being treatedseparately. If the engagement of the milling cutter is relativelyconstant through an element, one feed rate is used for that element. Ifthe engagement of the milling cutter varies through the geometryelement, the element is broken into segments and different feed ratesare applied to each based on the greatest engagement in that segment.

Preferably, the feed rate is adjusted to control the load on the millingcutter to be approximately equal to a predetermined value over a toolpath as long as the peripheral feed rate is within an acceptable range.

The UCT at a point along a curved tool path with a radius of curvature Ris found as:UCT=r−√{square root over (r ²−2fr cos(L)+f ²)} where:   (3)

L=K−E;

K=π/2−I/2;

I=2*sin⁻¹{f/(2*R)};

R=radius of curvature of the tool path at the point of calculation;

E=engagement angle at the point of calculation;

f=distance that a flute moves per revolution of the milling cutter, i.e.chip load; and

r=tool radius.

When the tool path is straight at the point of calculation, I=0 andequation 3 reduces to:

$\begin{matrix}{{U\; C\; T} = {r - \sqrt{r^{2} - {2\;{fr}\;{\cos\left( {\frac{\pi}{2} - E} \right)}} + f^{2}}}} & (4)\end{matrix}$

Preferably, the feed rate is controlled based on the value of the toolengagement and the UCT. Upon determining a shape of the tool path at aparticular point, all of the parameters necessary for computing the feedrate at the point are known to the CAM program 18. Accordingly, if theUCT is selected as the basis for feed rate control, at step 100.10 theUCT and the engagement are calculated for points along the path and theprogrammed feed rate of the milling cutter is controlled at the pointsalong the tool path to maintain the load on the milling cutter to beapproximately equal to a predetermined value (step 100.12).

Alternatively, the feed rate may be based on the peripheral feed rate ofthe milling cutter (steps 100.14 and 100.16). The instantaneousperipheral feed rate at points along a tool path having a radialcurvature R is related to the center-line feed rate by:

$\begin{matrix}{{{P\; F\; R} = {C\; F\;{R\left( {1 + \frac{r}{R}} \right)}\mspace{14mu}{for}\mspace{14mu} a\mspace{14mu}{concave}\mspace{14mu}{tool}\mspace{14mu}{path}}},} & (5) \\{{{P\; F\; R} = {C\; F\;{R\left( {1 - \frac{r}{R}} \right)}\mspace{14mu}{for}\mspace{14mu} a\mspace{14mu}{convex}\mspace{14mu}{tool}\mspace{14mu}{path}}},{{where}\text{:}}} & (6)\end{matrix}$

PFR=Peripheral feed rate; and

CFR=Center-line feed rate.

First Preferred Embodiment

When milling a tool path comprising a series of concentric circular arcsof different radius with the same milling cutter, the engagement of themilling cutter changes from one arc to the other if the radial depth ofcut (stepover) for each of the arcs is fixed at a single value. The sameis true of a spiral tool path having a constant rate of increase of theradius of the spiral.

A tool path suitable for milling a circular pocket or a circular island,consisting of one or more passes, may be created for which theengagement of the milling cutter following the tool path does not exceeda predetermined maximum value and does not deviate by more than apredetermined amount from a predetermined value over a major portion ofthe tool path by: (1) defining a plurality of concentric circles, inwhich a radius of each one of the concentric circles is determined suchthat an engagement of the milling cutter milling each one of theconcentric circles equals a target engagement which is substantiallyidentical to the maximum value of the engagement (FIG. 24) and, (2)arranging each one of the one or more passes between adjacent ones ofthe concentric circles such that each end of each pass is tangent to oneof the adjacent concentric circles (FIG. 25). The resulting generallyspiral tool path (FIG. 26) causes an engagement angle to gradually andsteadily rise from the starting position over a 360 degree path to apredetermined value, stay substantially constant at the predeterminedvalue for a majority of the tool path and steadily and graduallydecrease over a 360 degree path at the conclusion of the tool path. Notethat the target engagement may be incrementally less than the maximumengagement depending on the size of the boundaries as described below.

In the first preferred embodiment, the radii of the concentric circlesare determined as a function of the target engagement, the radius of anadjacent circle and the radius of the milling cutter. Where the adjacentcircle is larger, the next smaller circle is determined in accordancewith recursive equation 7a.R2=√{square root over ((R1−r)² +r ²−2r(R1−r)cos(π−E))}{square root over((R1−r)² +r ²−2r(R1−r)cos(π−E))}{square root over ((R1−r)² +r²−2r(R1−r)cos(π−E))} where:   (7a)

r=radius of milling cutter;

E=target engagement angle;

R1=radius of the larger circle; and

R2=radius of the next smaller circle.

Where the adjacent circle is smaller, the next larger circle isdetermined in accordance with recursive equation 7b.R2=r cos(π−E)+√{square root over (r ² cos² E−r ² +R ₁ ²)}+r where:  (7b)

E=the target engagement angle of the milling cutter;

r=the radius of the milling cutter;

R2=the radius of the next larger circle;

R1=the radius of the next smaller circle, and

The stepover between each of the first circular arcs is found byequation 8.S=R1−R2   (8)

The shape of each one of the arcs arranged between the concentriccircles is chosen such that the value of the engagement of the millingcutter traversing the tool path is either gradually and steadilyincreasing, substantially constant or gradually and steadily decreasing,based on the location of the arc in the tool path.

A connecting pass consisting of a first circular arc and a secondcircular arc that meets the above criteria may be constructed betweeneach pair of adjacent concentric circles by making one end of each ofthe first and the second circular arcs tangent to one of the adjacentconcentric circles and joining the opposite end of each of the first andthe second circular arcs to each other such that the pass thus formed istangent continuous at the point of the joined arcs.

FIG. 4 is a diagram of showing one method for constructing a blend, inthis case a two arc pass, where the two circular arcs have radii R₁ andR₂. The distance c, from the start point of the left hand circular arcto the endpoint of the right hand circular arc and the angles α and βare computable from geometry. The distances of the lines labeled a and bare unknown. The distance between the ends of lines a and b must be a+bbecause the two lines on the left must be the same length (namely, a)and the two lines on the right must also be the same length (namely, b).Letting a=b, equation 9 may be solved for a.2a ²[1−cos(α+β)]+2ac(cos α+cos β)−c ²=0   (9)

The join point of the circular arcs is at:

$\begin{matrix}{{\frac{1}{2}\left( {{a\;\cos\;\alpha} + c - {a\;\cos\;\beta}} \right)},{\frac{1}{2}\left( {{a\;\sin\;\alpha} + {a\;\sin\;\beta}} \right)}} & (10)\end{matrix}$

The distance between the join point and the start of the pass is h. Thisis the length of the chord of the leftmost segment that joins the endsof the two tangent lines of the leftmost arc, which can be computed froma, α, and β. The radius of the leftmost arc is then simply computed fromthe relationship of equation 11.

$\begin{matrix}{h = \frac{2{ar}}{\sqrt{a^{2} + r^{2}}}} & (11)\end{matrix}$

The radius of the rightmost arc can be determined similarly One skilledin the art would recognize that other blending techniques that wouldmaintain a tangent continuous tool path could be used.

Preferably, a median circle, having a radius equal to

$\frac{{R\; 1} + {R\; 2}}{2}$is defined between each pair of concentric circles, wherein each one ofthe one or more passes is arranged between one of the concentric circlesand one of the median circles such that each pass is tangent to aconcentric circle and to a median circle. Preferably, when constructinga tool path for a circular pocket comprising the set of passes, each oneof the passes is a semicircle, where the ends of each of the semicirclesare joined to each other such that the ends are tangent continuous atthe point of joining.

Preferably, the first semicircle in this set begins tangent to astarting-hole circle, and ends tangent to a circle whose radius is themedian of the radius of the starting-hole circle and the second (nextlarger) concentric circle in the set. The next semicircle begins tangentto the end of the first semicircle, and ends tangent to the secondconcentric circle in the set. The previous two steps are repeated untilthe set of semicircles is complete. The result is a tangent contiguouschain of semicircles extending from the radius of the starting hole tothe radius of the part boundary.

Preferably, the radius r and center offset C for each semicircle arefound from

$\begin{matrix}{{r_{k} = \frac{R_{k} + R_{k}^{\prime}}{2}};} & (12) \\{{C_{k} = \frac{R_{k}^{\prime} - R_{k}}{2}};} & (13) \\{r_{k + 1} = \frac{R_{k + 1} + R_{k}^{\prime}}{2}} & (14) \\{C_{k + 1} = {\frac{R_{k}^{\prime} - R_{k + 1}}{2}\mspace{14mu}{where}\text{:}}} & (15)\end{matrix}$

R_(k) is the kth concentric circle as numbered from the center of theconcentric circles and R_(k)′ is a virtual circle having a radius of

$R_{k}^{\prime} = {\frac{R_{k} + R_{k + 1}}{2}.}$

Typically, when milling a circular pocket, a starting hole is formed atthe center of the circular pocket and the milling proceeds from theradius of the starting hole to the part boundary. The series of materialboundaries as calculated by equation 7a may result in the radius ofmaterial boundary formed by the last circle to be computed (the innercircle) to not coincide with the radius of the starting hole. Similarly,if the series of material boundaries is calculated from equation 7b, thelast circle to be computed may not coincide with the part boundary.These occurrences could cause the engagement angle of the final millingcut to deviate excessively from the maximum engagement angle in order tomatch the last material boundary to the part boundary or could requirethe milling cutter to cut air at the beginning of the tool path.Consequently, in the first preferred embodiment, the target engagementof the milling cutter is marginally decreased to align the first and thelast concentric circles with the boundaries by (a) defining a largestone of the concentric circles on a first boundary, (b) determining if asmallest one of the concentric circles coincides with a second boundary;and (c) if the smallest concentric circle does not coincide with thesecond boundary within a predetermined value, decrementing the targetengagement and repeating steps b and c until the smallest concentriccircle coincides with the second boundary within the predeterminedvalue. Alternatively, the process for aligning the circles with theboundaries may be performed by working from the inside to the outside.

FIG. 5 shows a preferred process 200 for adjusting the target engagementangle so as to align the material boundaries milled by the millingcutter traversing the first and the last circular arcs with the partboundary and the boundary of the starting hole. At step 200.2, astarting point on the part boundary and an ending point on a boundary ofthe starting hole are determined. At step 200.4, a radius of millingcutter is defined. At step 200.6, a target engagement is determined forthe tool path based on the characteristics of the selected millingcutter and the type of material to be milled. At step 200.8, the radiusof the circular material boundary R2, adjacent to the part boundary R1,is computed based on equation 8. At step 200.10, the radius of R2 iscompared with the radius of the starting hole boundary. If the radius ofR2 is greater than the radius of the starting hole boundary, step 200.8is repeated using the radius of the last computed material boundary asthe basis for the computation until the radius R2 is less than theradius of the starting hole boundary.

At step 200.12 the radius of the material boundary formed by the millingcutter having a radius less than the radius of the starting hole iscompared with the radius of the starting hole. If the difference betweenthe radius of the starting hole and the radius of the material boundaryis less than a predetermined value, the circle radii are computed asoffset from each of the material boundaries by the milling cutterradius, and the radii are stored (step 200.1). If on the other hand, thedifference between the radius of the starting hole and the radius of thematerial boundary is greater than the predetermined value, the targetengagement is decremented by computing the difference (step 200.18)between the target engagement and the engagement angle of the lastmaterial boundary, E′ (step 200.16), decrementing the target engagementin proportion to the difference at step 200.20 and repeating steps 200.8to 200.18 until the difference between the last computed materialboundary and the starting hole boundary is within the predeterminedvalue.

Second Preferred Embodiment

Referring now to FIG. 6 there is shown an illustration of the generalcase where material to be milled has been left in a region of aworkpiece by a previous pass of the milling cutter. The region ofmaterial to be milled has first, second and third sides, the first sidebeing a current in-process material boundary and meeting at respectiveends, the second and the third sides. The second and third sidescomprise a part or an in-process material boundary of arbitrary shapeconsisting of a set of lines, arcs and splines. FIG. 7 illustrates atypical case in which the remaining material to be milled is in the fourcorners of a rectangular pocket.

A common characteristic of the above examples is that the in-processmaterial boundary is an arc that is substantially circular and greaterin radius than the radius of the active cutting tool. Preferably, themilling cutter to be used to mill at least a substantial portion of theremaining material is the same as the previous milling cutter, thusobviating the need for replacing the previous milling cutter for millingat least a substantial portion of the material to be milled.

The second preferred embodiment generates a tool path comprising one ormore advancing passes which successively advance into the region. Wherethe second and third sides are not parallel, the tool path is createdby: (1) defining a maximum engagement of the milling cutter; and (2)determining a radius of curvature and a center of the current in-processmaterial boundary; determining a radius of curvature and a center foreach of the advancing passes based on the radius of curvature and centerof the of the current in-process material boundary; an anglerepresentative of the intersection of tangents to the second and thethird sides, a radius of the milling cutter and the maximum engagement.The tangents are formed at the meeting points of the current in-processboundary with respectively the second side and the third side.Preferably, each advancing pass is tangent to both the second side andto the third side.

In general, the advancing passes do not have a common center.Consequently, the engagement angle may not be constant over a pass.Typically, the engagement angle reaches a maximum at a single pointalong the pass. Where the new in-process material boundary is tangent tothe two sides, the point of maximum engagement can be preciselydetermined. FIG. 8 a shows the determination of the point of maximumengagement. In such a case, the milling cutter is at the point ofmaximum engagement when the leading edge of its periphery reaches themidpoint of the in-process material boundary. The point of maximumengagement is at the same location whether the sides converge, diverge,or are parallel. FIG. 8 b shows that the engagement angle is smallerboth just before and just after this point.

Since the point of maximum engagement is known, the location and radiusof the next in-process material boundary can be calculated such that themaximum engagement angle is not exceeded.

The radius of curvature and the center of each advancing pass of thetool path is computable from knowledge of a radius of curvature and acenter of a current in-process material boundary, the angle of the partboundary and the radius of the milling cutter, where the angle of thepart boundary is defined as the included angle formed by theintersection of tangents to the part boundary at the points where thecurrent in-process material boundary intersects the part boundary.

Where the boundary is two converging lines each successive in-processmaterial-boundary radius is smaller than the previous radius and istangent to the boundaries. The tool path for milling each newlycalculated radius is a tool-radius offset from that radius. Where theboundaries diverge each successive in-process material-boundary radiuswill be larger than the previous radius and tangent to the boundary.Equations 16-18 provide a method for determining the radius and centerof each advancing pass. Alternatively, an iterative search method couldalso be used for finding the center and the radius of each successivein-process material boundary.

$\begin{matrix}{R_{2} = {\left( {\tan^{2}F} \right)\left( {B + \sqrt{B^{2} - {\left( \frac{1}{\tan^{2}F} \right)\left( {{2{r^{2}\left( {{{Cos}\; E} - 1} \right)}} + {R_{1}^{2}\left( {\frac{1}{{Sin}\; F} - 1} \right)}^{2}} \right)}}} \right)}} & \left( {16a} \right)\end{matrix}$where the boundaries are converging,

$\begin{matrix}{R_{2} = {\left( {\tan^{2}F} \right)\left( {B - \sqrt{B^{2} - {\left( \frac{1}{\tan^{2}F} \right)\left( {{2\;{r^{2}\left( {{{Cos}\; E} - 1} \right)}} + {R_{1}^{2}\left( {\frac{1}{{Sin}\; F} + 1} \right)}^{2}} \right)}}} \right)}} & \left( {16\; b} \right)\end{matrix}$where the boundaries are diverging,

$\begin{matrix}{B = {{r\left( {{{Cos}\; E} - 1} \right)} + {R_{1}\left( {\frac{1}{{Sin}^{2}F} - \frac{1}{{Sin}\; F}} \right)}}} & (17) \\{S = {\frac{{R\; 1} - {R\; 2}}{\sin\; F}\mspace{14mu}{where}\text{:}}} & (18)\end{matrix}$

-   -   r=the radius of the milling cutter;    -   F=the included half-angle formed by an intersection of tangents        to the part boundary at the point of intersection with the        current in-process mater boundary, where 0>F<90 degrees;    -   E=the maximum engagement angle;    -   R2=the radius of a new pass;    -   R1=the radius of the current in-process material boundary, and    -   S=the difference in centers along a median line between the        passes.

The second preferred embodiment is useful for expanding a starting holeinto a slot. FIG. 10 shows a starting circle expanding into a slot thathas sides that are parallel lines. In such cases, a series of 180-degreearcs is machined, in sequence, until the desired length of the slot isachieved. The radius of each of the 180-degree arcs is equal to theradius of the starting hole. The spacing S for the centers of theadvancing passes is based on the radius of curvature of the in-processmaterial boundary, the radius of the milling cutter and the maximumengagement. Equation 19 is a mathematical formula for finding therequired spacing. An iterative search method can also be used to findthe required spacing.S=R1−√{square root over (R1²−2rR1+2r ²+(2rR1−2r ²)cos E)}  (19)

In cases where the part boundary elements are other than two straightlines, iterative search routines may be used to determine the radius andcenter of each successive pass. The preceding mathematical formulas areused to determine the engagement that results from each repetition ofthe steps in the iterative search routine.

FIG. 11 shows the first three steps in this iterative search. Twoconstruction lines are created that are tangent to the two boundary arcsin FIG. 11. The construction lines intersect at an angle equal to 2 F.degrees. For calculation purposes, the construction lines serve as thelines in the converging-lines case described above. Equations 16-18 maybe applied to find an in-process material boundary. A key difference inthe case of non-linear part boundaries is that the point of maximumengagement does not occur where the periphery of the tool reaches themidpoint of the current in-process material boundary. Rather, the pointof maximum engagement occurs when the periphery of the milling cutterreaches the intersection point between the current in-process materialboundary and the bisector of the angle formed by the two constructionlines. If the arc, which is tangent to the two construction lines, notthe two boundary arcs, is sufficiently close to the tangent arccandidate, the candidate arc is used as the next in-process materialboundary. If the arc is not sufficiently close, then the radius of thearc found with the formula is used to create the next candidate arc,tangent to the two boundary arcs.

FIG. 12 shows a continuation of the iterative process. The midpointobtained after using the formula is used to create a new arc, tangent tothe two boundary arcs. The process is repeated until the required nextin-process material boundary is found.

FIG. 13 illustrates using the method of the second embodiment togenerate the passes required to machine a region in which the boundaryentities first converge and then diverge, without exceeding the maximumtool engagement angle. In some cases, one pass is all that is requiredto machine an area. In other cases, multiple passes are required. Thereturn lines 3, in FIG. 13 illustrate one way to connect the multiplepasses. In generating the tool path required to machine multiple passes,any of several known methods can be used to connect between the multiplerequired passes.

Third Preferred Embodiment

Referring to FIG. 14, there is shown a workpiece in which a step, whichis open on two sides, is to be milled. A conventional method for millingsuch geometry is that of milling in parallel, straight lines (directionparallel strategy). However, milling in parallel straight lines toward apart or a material boundary results in a large increase of theengagement of the milling cutter as the milling cutter approaches theboundary. Consequently, spindle speeds and/or feed rates for the passesmust be set for the highest possible engagement (up to 180 degrees),which limits the efficiency of material removal.

To avoid excessive engagement of the milling cutter, the method of thethird preferred embodiment generates a tool path of one or more parallelpasses separated by a predetermined value of a stepover. The shape ofthe tool path is based on the radius of the milling cutter to be usedfor milling, and a maximum engagement of the milling cutter. Each passcomprises a first arc and a tangent continuous second circular arc. Thefirst arc of each pass is typically straight, but may be arcuate. Thein-process material boundary formed by the end of the second arcopposite to the end joining the first arc is tangent to the part or amaterial boundary and has a radius selected such that the engagement ofthe milling cutter traversing the tool path becomes equal to and doesnot exceed the maximum engagement.

Since each first arc blends into a circular arc of a known radius, theengagement angle during any given pass is greatest at the point wherethe first arc joins the second arc. Consequently, the maximum engagementangle is selected based on the radius of the second arc. Further, sincethe first arcs are generally parallel to each other, the starting angleof the second circular arc of all the passes is identical. Consequently,equation two can be used to determine the value of the stepover betweenthe passes. It follows then, that the radius of the second circular arcdetermines the actual spacing (stepover) between the passes.

The radius of the second arc can be virtually any size the user desires,since any radius larger than the radius of the milling cutter will work.Preferably, the radius of the second arc is between two and six timesthe radius of the milling cutter. More preferably, the radius of thesecond arc is approximately four times the radius of the milling cutter.

Fourth Preferred Embodiment

When a milling cutter enters the workpiece from outside an in-processmaterial boundary and attains a maximum engagement of the millingcutter, there is one and only one path that the milling cutter canfollow and maintain without exceeding the maximum engagement while alsomaintaining its cutting direction (climb/conventional).

The fourth preferred method is a method for generating a tool pathhaving a maximum engagement when the tool path enters the workpiece fromoutside a material boundary at a predetermined attitude. The resultingtool path is referred to hereafter, as a tool engagement curve. Themethod includes generating, by a direct process, a tool path consistingof one or more passes. The method includes defining a maximum engagementof the milling cutter and defining each one of one or more passes suchthat a value of the engagement, when traversing each one of the one ormore passes, does not exceed a maximum value of engagement. In thefourth embodiment, the tool path is determined by first locating astarting point inside the in-process material boundary such that thevalue of engagement of the milling cutter, when located at the startingpoint is substantially equal to the maximum engagement; thensuccessively locating each of one or more points inside the in-processmaterial boundary such that the value of engagement of the millingcutter, when located at each one of the one or more points, issubstantially equal to the maximum value of engagement, and thenarranging the one or more passes between the one or more successivepoints to thereby connect together the starting point and each one ofthe successive one or more points to form the tool path.

Preferably the location of each of the one or more successive points isfound by establishing the initial attitude angle perpendicular to thetrailing edge of the engagement angle, rotating the attitude of the toolpath to decrease the engagement in the next leg of the tool path. Thecutter can now be moved further into the material at this new attitudeuntil the leading edge of the engagement angle contacts the materialboundary, thereby establishing the location of the next point.Preferably, the leading edge of the engagement angle is temporarilyadvanced by a factor of ½ of the rotation of the attitude at each step.However, the leading edge may be advanced using a different factor ofthe rotation. Further, it is unnecessary to advance the leading edge ofthe engagement at each step and the tool path generated by the fourthembodiment would still generate a tool path having an engagement lessthan the maximum value.

Preferably the location of each of the one or more successive points isfound by projecting a first straight line from a preceding point, thefirst line being at a first angle with respect to the attitude of thetool path at the preceding point; projecting a second straight line fromthe preceding point, the second line being at a second angle withrespect to a leading edge of the engagement angle of the milling cutter,the second angle being equal to one-half the first angle; determining anintersection of the second straight line projection with the in-processmaterial boundary; and determining an intersection of a straight linehaving a length equal to a radius of the milling cutter and originatingat the an intersection of the second straight line with the in-processmaterial boundary with the first line.

An example of constructing the tool engagement curve in accordance withthe fourth embodiment is shown in FIG. 15. In FIG. 15, the path of amilling cutter is shown approaching the in-process material boundary ona linear path that is normal to the boundary with the object of reachinga specified radial depth of cut in the shortest distance consistent witha maximum engagement angle. In FIG. 15, the tool path proceeds into thematerial until the milling cutter reaches the maximum engagement. It isapparent that the tool path cannot move farther in a directionperpendicular to the in-process material boundary without the millingcutter exceeding the maximum engagement. However, by rotating theattitude of the tool path, defined as being perpendicular to thetrailing leg of the engagement angle, by an amount, the milling cuttercan be moved farther into the material in a new direction withoutexceeding the maximum engagement. This is because, as the milling cutteris rotated into the new attitude, the engagement angle is decreasedmomentarily, enabling the milling cutter to be moved forward along thetool path until the maximum engagement angle is once again reached.

Preferably the above described steps are repeated, with the tool pathattitude being incremented and the tool moved along each new attitude,enabling the tool to proceed farther and farther into the material.Preferably, the amount of rotation is decreased as the tool approachesprogrammed radial depth of cut. Decreasing the amount of rotation as thetool approaches programmed radial depth of cut allows the spacingbetween the points to remain relatively constant.

Preferably, a smooth tool path is defined by fitting a spline curve tothe tool path such that the value of engagement of the milling cuttertraversing the tool path does not deviate from the maximum value by morethan a predetermined amount.

The entry attitude angle is not required to be normal to the surface touse the method of the fourth embodiment. The process of the fourthembodiment can be employed if the tool initially enters the material atany angle, or along a curved path. The process of the fourth embodimentalso provides a means for entering material from the side whilecontrolling the engagement angle. Further, the in-process materialboundary need not be straight but can be any shape. In addition, whilethe method of defining the points along the tool path has been stated asa geometrical construction, the method is not limited to a geometricalconstruction. A mathematical formula can be also used in place of theconstruction method for computing the points along the tool path.

Alternately, a variation of the above described embodiment can be usedto remove material that remains between the in-process boundary and apart boundary. The only difference in this case is that the partboundary must not be gouged. Therefore, when the tool reaches the partboundary in the process of proceeding toward full engagement, the toolfollows the part boundary, and continues to follow the part boundaryuntil the tool reaches a point when engagement can again be increased.In this manner a tool can be driven around an entire part boundary orany portion of it in order to remove remaining material withoutexceeding engagement. If engagement is not reached in some sections ofthis pass, the process can be repeated, as necessary.

A further use for the method of the fourth embodiment is the creation ofa tool path for milling a circular pocket or a circular island.Preferably, the attitude of the tool path entering the workpiece istangent to the material boundary, but it could be at any angle to it.The method comprises generating a tool engagement curve from the entrypoint into workpiece such that a spiral-like arc is formed. By extendingthe tool path around until it is reaches the beginning of the in-processmaterial boundary and then following it, a continuous spiral-like toolpath is created by repeatedly defining the location of successive pointsinside the material boundary such that the value of engagement of themilling cutter, when located at each point, is substantially equal tothe maximum value of engagement and arranging the passes between thesuccessive points, the tool path may be extended indefinitely eitherinwardly or outwardly in a generally spiral path such that the value ofthe engagement of the milling cutter is substantially constant and doesnot exceed a predetermined maximum value.

Fifth Preferred Embodiment

To accurately machine a pre-existing boundary, the tool path of the lastcut along that boundary must be at a tool-radius offset from thatboundary. However, if an engagement angle is to be held substantiallyconstant during the last cut, the in-process material boundary that ispresent prior to the commencement of the last cut must be of a specificshape, i.e., the in-process material boundary must be closer to the partboundary where the part boundary is concave and farther from the partboundary where the part boundary is convex. This is illustrated in FIG.16, which shows the in-process material boundary that must exist if thefinal cut, also shown, is to maintain a substantially constantengagement angle.

Referring now to FIGS. 16-19, there is described a fifth preferredembodiment of a method for generating a tool path for forming anin-process material boundary adjacent to a part boundary where the valueof the engagement angle of a milling cutter milling the part boundarydoes not deviate from a maximum engagement angle by more than apredetermined amount when traversing the tool path. FIGS. 16-17illustrate a graphical construction method that can be used to generatean in-process material boundary that results in a substantially constantengagement angle when cutting a part boundary (or any material boundary)of arbitrary shape. A normal, of tool-radius length, is generated ateach of a plurality of points arranged along the part boundary. Thenormals are then rotated through the engagement angle. Generating asmooth curve through the endpoints of these rotated normals produces therequired in-process material boundary. For any given point on the partboundary, a single corresponding point exists on the in-process materialboundary. Those skilled in the art will recognize that the accuracy ofthe result is dependent upon the number of part-boundary points used,with an increased number of points producing a more accurate curve.

In general the engagement offset method for determining the tool pathfor milling the material boundary immediately adjacent to the partsboundary, includes the steps of: selecting a plurality of first pointsalong the parts boundary; determining a plurality of second points, eachsecond point being located: (1) at a distance L=N sqrt (2−2 cos E) froma corresponding first point, and (2) at an angle of (pi−E)/2 from anormal to the parts boundary at the corresponding first point, E beingthe engagement angle and N being a predetermined length of the normal;and generating a smooth curve which intersects a plurality of thirdpoints, each of which third points is offset from a corresponding one ofthe second points and which is perpendicular to a smooth curve whichpasses through the second points.

There are other viable methods of generating the in-process partboundary. FIGS. 18 and 19 illustrate a method of “driving” a geometricentity along the part boundary while maintaining a constant orientationof that entity with a line tangent to the part boundary at each point.In FIG. 18, the driven entity is the engagement chord. In FIG. 19, thedriven entity is the leading side of the engagement angle.

As described, the method of the fifth embodiment generates thein-process material boundary required for machining a known boundary ata constant tool-engagement angle. It is possible, using iterativeprocedures, to do just the opposite—that is, for any given in-processmaterial boundary, a tool path can be calculated that removes materialfrom that boundary while maintaining a substantially constantengagement. This requires calculating the next in-process materialboundary and generating a tool path that produces that next in-processmaterial boundary.

Preferably, a first trial tool path is generated by constructing normalsalong the existing in-process material boundary and creating linesparallel to and offset from each normal by an amount that represents thecontact point of the leading edge of the milling cutter. From thecontact point, an engagement angle line is projected back to, andintersecting, a normal. The intersection of the engagement angle lineand the normal creates a tool-center point for a first trial tool path.The point where the tool radius intersects the normal creates a pointfor the first trial next-in-process material boundary. A spline, S1, isfitted through the series of points depicting the first trial boundary.

If the first trial next in-process material boundary were accurate,generating a curve according to the method of the fifth embodiment fromthe in-process material boundary would recreate the original in-processmaterial boundary. With this in mind, and referring to the method of thefifth embodiment, it can be seen that a normal, equal in length to theradius of the tool, projected from a point on this next in-processmaterial boundary defines the location of the center of the tool. Theend point of a vector from this center point, equal to the tool radiusand at an angle necessary to define the engagement angle, reveals theaccuracy of this trial next-in-process material boundary. Unless theoriginal in-process material boundary is a straight line or a radius,the vectors need to be adjusted by an iterative process as follows:

The iterative process shifts the original vectors that were normal tothe original in-process material boundary such that they are coincidentwith the projected normals from the trial next-in-process materialboundary. Note that when these vectors are rotated they will no longerbe normal to the original in-process material boundary. Then thetool-center point is moved up or down a normal projected from the trialin-process material boundary until the leading edge of the tool vectorrests on the original in-process material boundary.

The process creates a series of points where the tool radius intersectsthe normal from the next in-process material boundary. A spline, S2, isfitted through these points, creating a second trial next-in-processmaterial boundary. For the next iteration, new normals are projectedfrom S2 and the process is repeated. Each time the normals from the newnext-in-process material boundary shift, the tool representation,consisting of a tool radius vector and attached edge-engagement vector,is shifted so that the end of the engagement vector rests on theoriginal in-process material boundary. Iterations are repeated until thenecessary degree of accuracy is attained.

Sixth Preferred Embodiment

Referring now to FIGS. 20-23 there is shown a sixth preferred embodimentfor generating a tool path for a region defined by a closed boundary.The method includes the steps of: creating a family of concentricindexed circles at each of selected points within the region; connectingtogether the circles of adjacent families of circles having an identicalindex to form isoloops; and forming a spiral like path by blendingbetween the isoloops.

The first step for generating the tool path is to find points in theregion that are: (1) equidistant from three or more boundary points(branch points); (2) in the middle of a constricted area of the pocket(neck points) and (3) as close to the boundary as one can get with theselected milling cutter (corner points). Preferably, the well knownMedial Axis Transform (MAT) is suitable for finding the above pointsalthough other methods could also be used for finding the branch points,neck points and corner points. FIG. 20 illustrates the result ofexecuting a MAT for a trapezoidal region.

Each branch point, corner point and neck point is a candidate forlocating a family of concentric circles. Each circle family consists ofone or more concentric circles. The radius of each one of the concentriccircles is such that an engagement of the milling cutter milling eachone of the concentric circles is equal to a target value substantiallyequal to the maximum value of engagement. Preferably the radii of thecircles are computed using one of the procedures of the first preferredembodiment. As described in connection with the first preferredembodiment, the target engagement may be incrementally different fromthe maximum engagement depending on the dimensions of the boundaries.Circle families are generated at each branch point. Circle families aregenerated at neck points and corner points, depending on certainparameters such as the width of the neck, the size of the millingcutter, etc. The circle families are joined together in a graphstructure called a circle graph (see FIG. 21).

After creating the circle graph, the circles of each family are indexedby numbering the circles from the outside in an ascending scale. Thecircles having the same index are then joined together into isoloops, socalled because the isoloops are created by joining circles of the sameindex together. The joining process creates some combination of arcs(pieces of the original circles) and blends joining those piecestogether. FIG. 22 is an illustration of the circle families being joinedby the isoloops.

Preferably, the lines joining two circles together are constructed by:constructing two offsets from the boundary, c1(t) and c2(t) where c1 isa distance R1 from the part boundary and c2 is a distance R2 from thepart boundary. A blending curvec(t)=(1−2t^3+3t^2)*c1(t)+(3t^2−2t^3)*c2(t) is then constructed.Inspection of c(t) shows that the position and first derivatives matchc1 at t=0 and c2 at t=1. That means that c(t) is actually a tangentcontinuous blend. c(t) is not generally a circular arc. Consequently,c(t) is approximated with a tangent continuous sequence of circular arcsusing a two arc pass according to the first embodiment. The two arc passis fitted to c(t) by measuring the error between the circular arcs andc(t) and halving the value of c(t) recursively until the error isacceptably small. One skilled in the art would recognize that otherblending techniques that would maintain a tangent continuous tool pathcould be used to connect the circles.

A spiral-like tool path (FIG. 23) is then created by blending betweensuccessive isoloops. The blends are generated using the same exactblending function used for c(t) above. The only difference is that c1(t)and c2(t) are the isoloops instead of offsets. The blending paths arereplaced with sequences of arcs using the previously described recursivealgorithm.

Seventh Preferred Embodiment

The first preferred embodiment describes a method for creating agenerally spiral tool path for milling a circular region in which thevalue of the engagement of the milling cutter does not deviate by morethan a predetermined amount over a major portion of the tool path. Themethod uses pairs of semicircles which are non-concentric with respectto the center of the circular geometry. The region may be defined by theboundaries of a pocket or the boundaries of an island.

Referring now to FIGS. 31 and 32, there is shown a seventh preferredembodiment for creating a tool path for a circular region which usessemicircles to create a generally spiral tool path. The seventhpreferred embodiment differs from the first preferred embodiment in thatthe engagement of the milling cutter is not maintained constant within apredetermined amount over a major portion of the tool path but does havethe property that the tool engagement is held to a value less than apredetermined maximum value.

In the seventh preferred embodiment, the region is bounded by inner andouter concentric circular boundaries. The method for generating the toolpath comprises the steps of defining a first set of passes, the firstset of passes being semicircles, each one of the semicircles in thefirst set having a center at a center of the inner and the outerconcentric circular boundaries; and a second set of passes, the secondset of passes being non-concentric semicircles, each one of thesemicircles in the second set having a center offset from the center ofthe concentric circular boundaries by an amount greater than or equal toan amount of an adjacent inner non-concentric semicircle and/or lessthan or equal to an adjacent outer non-concentric semicircle, whereinthe amount of the offset and a radius of each one of the semicircles inthe second set of passes and the radius of the passes in the first setof passes is based on the maximum engagement, and wherein the first setof passes and the second set of passes are joined in an alternatingtangent continuous fashion to form a generally spiral tool path forremoving material between the inner boundary and the outer boundary.

When any individual semicircle of the set of concentric semicircles istraversed by the milling cutter, the engagement angle will be constantduring the machining of that semicircle, but the engagement angle willnot necessarily be the same for each of the concentric semicircles. Whenany individual offset semicircle is traversed by the milling cutter, theengagement angle will increase until it reaches a point of maximumengagement, but the engagement will not exceed the maximum. Preferably,each semicircle in the concentric set is generated based on an endpointof a corresponding semicircle from the offset set. The radius and thecenter point of each offset semicircle tool pass can be calculated fromthe radius and center point of the circular in-process material boundarycreated by that tool pass. Knowing the radius of the cutting tool, theradius of the outer boundary, or the radius of the in-process materialboundary created by the next inner concentric tool pass, the maximumengagement angle, the radius and center point of the in-process materialboundary created by each offset semicircle can be calculated. Startingat the outer boundary (FIG. 32 a), the radius and center point of thein-process material boundary created by the first offset semicircle passcan be calculated by the equation shown in FIG. 31, where the offset isfound from the difference between a radius of the outer boundary and aradius of the in-process material boundary adjacent to the outerboundary. The corresponding first offset semicircle tool pass thatcreates the adjacent in-process material boundary is created as a toolpass radius offset from this in-process material boundary.

The first semicircle in the concentric semicircle set can now beconstructed, tangent to the first offset semicircle tool pass at itsendpoint and concentric to the circular outer boundary. The equation inFIG. 31 is used again to calculate the next offset semicircle, based onthe in-process material boundary created by the first concentricsemicircle rather than the outer boundary. The next concentricsemicircle is then constructed based on this offset semicircle. Theprocess is repeated until the next offset semicircle tool passintersects the circular tool pass that created the inner circularboundary. When that occurs, a final 180-degree offset semicircle isgenerated between the final concentric semicircle (the smallestconcentric semicircle) and the circular tool pass that created the innercircular boundary. This results in one tangent continuous set ofsemicircle tool passes that remove the material between the innerboundary and the outer boundary that can be traversed by the millingcutter while never exceeding the maximum engagement angle.

A similar equation as that shown in FIG. 31 can be used to calculate thesemicircle sets in the opposite direction, from the inner boundary holetowards the outer boundary. (See FIG. 32 b). In this case, calculationscontinue until an offset semicircle is generated that intersects thecircle that is a tool radius offset from the outer circular boundary.When that occurs, a final 180-degree offset semicircle is generatedbetween the final concentric semicircle (the largest concentricsemicircle) and the tool radius offset of the outer boundary.

When the chain of passes is machined, the offset semicircle that wasconstructed last will be machined at a lesser engagement angle. Sincethis last-calculated offset semicircle will be shorter in length if thesemicircle sets are calculated from the outer boundary towards the innerboundary, this method of calculation will produce a more efficient toolpath. In either case, a final cut must be generated along the partboundary to ensure the complete removal of material.

It is also possible to arrive at the radii and center points of theoffset arcs using iterative search routines. The transition from thelast offset arc to the inner or the outer boundary can be smoothed byadding a blend. Alternatively, the maximum engagement can be decreasediteratively, using the procedure of the first embodiment to match thesets of semicircles smoothly to the inner and outer boundaries.

Eighth Preferred Embodiment

The second preferred embodiment discloses a method for expanding astarting hole into a slot based on a series of advancing passes, wherethe shape of each in-process material boundary resulting from each passof the milling cutter is a substantially circular 180 degree arc.However, as shown in FIGS. 33 and 34, when each advancing pass has theshape of a substantially circular arc, the engagement of the millingcutter remains high for approximately the first half of the pass, butfalls off rapidly in the latter portion of the pass.

Referring to FIG. 35 there is shown a tool path consisting of a seriesof passes 340 and a series of in-process material boundaries resultingfrom one or more passes 340 of a milling cutter which advances accordingto a predetermined pitch to mill a slot in accordance with the eighthpreferred embodiment. Preferably, each pass 340 results in an in-processmaterial boundary comprising a plurality of tangential continuousconnected arcs including: (1) a first arc 350, tangent to a first sideof the slot and having a radius of curvature greater that one-half thewidth of the slot, and (2) a second arc 320, tangent to a second side ofthe slot having a radius less than one-half the width of the slot andgreater than the radius of the milling cutter. Preferably, each pass 340consists of two circular arcs. However, the arcs need not be circularnor are the number of arcs in a pass 340 limited to two arcs. The eighthpreferred embodiment, as described below, achieves higher materialremoval rates and improved efficiency compared to the method of thesecond preferred embodiment by maintaining a large value of theengagement of the milling cutter over a greater portion of each pass,thereby removing more material during each advancing pass.

In the eighth preferred embodiment, a ratio of the slot-width to themilling cutter radius is preferably in the range of 2.2 to 5. Howeverthe ratio of the slot-width to milling cutter radius may be any valuegreater than two. Preferably, the second arc 320 has a radiusapproximately equal to the sum of the smallest radius of the tool pathand the radius of milling cutter, and preferably, has an arc length ofbetween ninety degrees and one-hundred twenty degrees.

Where the width of the slot is less than or equal to three times theradius of the milling cutter, the radius of the first arc 350 ispreferably made approximately equal to three times the radius of themilling cutter minus the radius of the second arc 320, and preferably,an arc length of approximately ninety degrees. Where the width of theslot is greater than three times the radius of the milling cutter, theradius of the first arc 350 is preferably made approximately equal tothe width of the slot minus the radius of the second arc 320, andpreferably, the length of the first arc 350 is made 90 degrees or less.However, the radius of the first arc 350 may be any value greater thanone-half the width of the slot and the lengths of the first arc 350 andthe second arc 320 could be larger or smaller than the preferred rangesand still be within the spirit and scope of the invention.

Preferably, the value of the pitch, and the radius of the first arc 350and the radius of the second arc 320 are determined such that themaximum engagement of each pass 340 is substantially equal to thepredetermined maximum value of the engagement.

Preferably, the pitch “p” is determined by the computer program or theequivalent steps shown in Table I by first selecting a value “E” for themaximum engagement of the milling cutter, a value “r” for the radius ofthe milling cutter, and a value “r4” representative of the smallestradius of the tool path, generally set to one tenth of the radius of themilling cutter. However, one skilled in the art would recognize that theprogram of Table I is only one of a number of ways of calculating thepitch. One skilled in the art would understand that the program of TableI is derived from the relationships inherent to the geometry of themultiple arc material boundary bounded on each end by a part or materialboundary. Accordingly, other methods for determining the radius and thearc lengths of each portion of a multiple arc material boundary based onthe inherent geometry are within the spirit and scope of the invention.

TABLE I LABEL INPUTDATA INPUT “Enter radius of tool, r ”, r INPUT “EnterEngagement angle in deg., E ”, E INPUT “Enter Width of Slot, W ”, WINPUT “Enter Smallest TP radius, r4 ”, r4 LABEL Start R2=r+r4 IFW>(3*r)THEN ALT LABEL V1 R1=(2*r)−r4 R3=R1−r Ra=Sqrt(R3{circumflex over( )}2+r{circumflex over ( )}2−2*R3*r*Cosine(180−E)) b=R1−R2 c=R1+R2−Wa=Sqrt(b{circumflex over ( )}2−c{circumflex over ( )}2) A1=Acosine(c/b)C1=90−A1 h=Sine(C1)*r4 f=Cosine(C1)*r4 J=E−C1 D1=180−J k=Sine(D1)*rk=ABS(k) n=Cosine(D1)*r m=k−h g=n−f t=SQRT(R2{circumflex over( )}2−2M{circumflex over ( )}2) p=g+t Ae=Acosine((a−g)/Ra)Ar=Acosine(t/R2) IF AE>Ar THEN ECIRCLE GOTO END LABEL Ecircle R5=R3−r4d=W−2*r−2*R4 f=R5−d g=Sqrt(R5{circumflex over ( )}2−f{circumflex over( )}2) R6=R2−Ra h=Sqrt(R6{circumflex over ( )}2−f{circumflex over ( )}2)p=g+h GOTO End LABEL ALT R2=r+r4 R1=W−R2 Ra=Sqrt(R3{circumflex over( )}2+r{circumflex over ( )}2−2*R3*r*Cosine(180−E)) R6=R1−Ra p=R6 LABELEnd INPUT “Find new pitch? n ends program     ”,A$   IF A$ = “n”  CLOSECONSOLE     END   ELSE   GOTO INPUTDATA   ENDIF

The value of pitch determined by steps in Table I does necessarilyresult in the largest possible value of pitch for which the maximumengagement is substantially equal to the desired maximum engagement.FIG. 36 is a flowchart which describes an iterative process 300 whichmaximizes the pitch for given values of engagement and milling cutterradius. In step 300.2, the desired maximum engagement E of the millingcutter is selected. At step 300.4, the radius r of the milling cutter isselected. At step 300.6 an initial value of the minimum radius of thetool path is selected for the purpose of varying the values of R1 andR2. At step 300.8, a calculation is made, using the steps of Table I, ofan initial value of pitch for achieving the desired maximum engagement.At step 300.10, the maximum value of pitch which results in the maximumengagement being substantially equal to the desired maximum engagementis determined by adopting trial values for r4 around the initial valueof r4, computing the pitch for each of the trial values of r4 using thesteps of Table I and, by using any one of known techniques, estimatingthe maximum value of the pitch from the values of pitch calculated fromthe trial values of r4. It should be understood that the program ofTable I is not the only way for determining the pitch. Other methodsincluding graphical construction methods are also suitable.

The eighth embodiment is not limited to generating a tool path formachining a slot. In general, the method of the eighth embodiment isapplicable to any region having first, second and third sides, where thefirst side is a current in-process material boundary, the second andthird sides are part or material boundaries and the first side meets atits respective ends, the second and the third sides. For example, themethod of the eighth embodiment can be applied to machining converging,parallel, and diverging configurations of material boundaries includingan “hour glass” shape similar to FIG. 13.

An example of determining the parameters of a tool path for machining acorner in accordance with the eighth embodiment is shown in FIG. 37,where the line identified as side 1 is the current in-process materialboundary and sides 2 and 3 correspond to part boundaries. According, toprocess 300, values for the tool radius, and the radii of the first andsecond arcs, R1 and R2 and the pitch, are selected for achieving adesired maximum engagement using the principles described above andusing as the width, the distance between the two sides at theintersection of the in-process material boundary and the sides. A centerfor the second arc (smaller arc) is selected within the region to bemilled which results in the second arc being tangent to the side 2. Acenter for the first arc is then determined such that the first arc istangent to the side 1 and tangent continuous with the second arc. Theprocess of advancing each pass (radii R1′ and R2′) is then continued,taking into account the varying width of the region.

Ninth Preferred Embodiment

When milling a pocket using an inside-out tool path, it frequentlyoccurs that the milling cutter traverses corners in which the engagementof the milling cutter increases as compared to the engagement of themilling cutter when traversing portions of the tool path adjacent to thecorners. Such increases in the milling cutter engagement requiredownward adjustment of the feed rate in order to avoid excessive millingcutter wear and other problems, as discussed previously. In the ninthembodiment, the tool path is modified to introduce preparation cutsprior to machining each corner in the tool path. Consequently apredetermined engagement is not exceeded when machining a preparedcorner.

Referring now to FIG. 38, there is shown an in-process material boundarycorresponding to an inside-out tool path for machining a pocket inaccordance with the ninth embodiment. In the ninth preferred embodiment,the in-process material boundary comprises a plurality of first portions402 a, a plurality of second portions 402 b and a plurality oftransition portions 404 connecting together the plurality of firstportions 402 a and the plurality of second portions 402 b. While thefirst and second portions 402 a, 402 b are shown to be straight lines inFIG. 38, the first and second portions 402 a and 402 b need not bestraight lines but may be arcuate. However, in practicing the ninthembodiment, the transition portions 404 generally have a greatercurvature than the first and second portions 402 a, 402 b.

Generally, the plurality of first portions 402 a are parallel to eachother and the plurality of second portions 402 b are parallel to eachother. The plurality of first portions 402 a and the plurality of secondportions 402 b are separated by a common value of stepover 406. WhileFIG. 38 shows a rectangular pocket in which the in-process materialboundary is spiral-like extending from a slot shaped starting hole 416to a part boundary, one skilled in the art would understand that theninth embodiment is not limited to a rectangular part boundary with aslot shaped starting hole and having a tool path with straight lines andthat both the part boundary, the starting hole and the tool path couldbe completely arbitrary in shape and still be within the spirit andscope of the invention.

Preferably, the stepover 406 between the first portions 402 a and thesecond portions 402 b is determined based on a predetermined maximumengagement of the milling cutter when machining the first and the secondportions 402 a, 402 b. Preferably, the stepover 406 results in theportion of the in-process material boundary adjacent to the partboundary of the pocket being coincident with the part boundary. Wherethe stepover 406 results in non-coincidence of the in-process materialboundary with the part boundary, a process similar to process 200, shownin FIG. 5, may be used to adjust the value of the stepover 406 to makethe portion of the in-process material boundary coincident with the partboundary. Alternative to determining the value of the stepover 406 basedon a predetermined maximum engagement, the stepover 406 may be selectedbased on, for example, experiment or experience, and the maximumengagement determined from the selected value of stepover 406 usingequation 2.

Referring now to FIG. 39, there is shown a detail of the in-processmaterial boundaries formed by the transition portions 404 of the toolpath. Initially, the parameters for each transition portion 404 arefound by: (1) determining a radius 412 of a first arc 408 based on thestepover 406 and a width 414 of the starting hole 416, and (2)determining a radius 418 of a second arc 410 for each of the transitionportions 404 based on the radius 412 of the first arc 408 and themaximum engagement of the milling cutter.

Preferably, the radius, (Rt), 412 of the first arc 408, equals the sumdivided by two of the width 414 of the starting hole 416 and thestepover 406, and the radius 418, (Rp1) of the second arc 410 equalssqrt (r^2+(Rt−r)^2−2*r*(Rt−r)*Cos(180−E)), where E is the maximumengagement angle and r is the radius of the milling cutter.

FIG. 40 shows the development of the in-process material boundary forthe transition portion 404 of the tool path. Preferably, the tool pathfor each transition portion 404 is determined by situating the first arc410 so as to connect the first portion 402 a and the second portion 402b in a tangent continuous manner; situating a third arc 420 so as to betangent continuous with a first end 408 a of the first arc 408 at afirst end 420 a and intersecting a second end 408 b of the first arc ata second end 420 b; and situating a fourth arc 422 so as to be: (1)tangent continuous with the second end 420 b of the third arc 420, (2)tangent to the second arc 408 and (3) tangent continuous with at leastone of the first portion 402 a and the second portion 402 b.

Preferably, the tool path for each transition portion 404 is generatedby sequentially computing the parameters of the first arc 408, the thirdarc 420 and the fourth arc 422 for each transition portion in turn asthe tool path for the pocket is developed. However, it is not necessaryto compute the parameters in that order. For instance, the parameters ofthe first arc 408 could be computed as an initial spiral-like tool pathfor the pocket is computed and the parameters for the third and theforth arcs computed after the initial tool path is computed to arrive atthe final tool path. Further, one skilled in the art would recognizethat the insertion of a transition portion 404 into a tool path is notlimited to a spiral-like tool path. A transition portion 404 could beinserted into a portion of any shape of parallel offset tool path whereit would be desirable to reduce the engagement of the milling cutter inthat portion.

Tenth Preferred Embodiment

Referring now to FIG. 41 there is illustrated a tool path for enlarginga region bounded by a first material boundary 602. Also shown is a toolpath 604 consisting of one or more 360 degree circular passes ofincreasing radius arranged end to end. A first pass 604 a and a secondpass 604 b are shown. FIG. 41 also shows a second material boundary 606a created by the first pass 604 a and a third material boundary 606 bcreated by the second pass 604 b at an offset equal to the radius 608 ofa milling cutter. Each of the one or more passes 604 are tangent at acommon point and each material boundary 606 created by the one or morepasses 604 are tangent at a common point.

If a radius of each one of the one or more circular passes is properlyselected, the first material boundary 602 may be enlarged by a tool path604 consisting of the one or more circular passes such that anengagement of the milling cutter, when following the tool path 604, willnot exceed a predetermined engagement.

Referring to FIG. 42, an engagement circle 610 having a radius Re can bedefined as a circle traced by the end point of a leading side of apredetermined tool engagement angle as a milling cutter of radius Rt,608 traverses a circular pass (not shown) of radius Ra for creating anenlarged material boundary 606 b having a radius of Ra+Rt. For a givenvalue of Ra, a given value of the tool engagement angle E, and a givenvalue of the radius Rt, 608, the value of Re of the correspondingengagement circle 610 can be calculated using basic trigonometricformulas as:Re=√{square root over (Ra ² +Rt ²+2RaRt cos E)}  (20)

Characteristically, the engagement of a milling cutter following a pass604 gradually increases toward a maximum value and then decreases. Ifthe engagement circle 610 intersects a previous material boundary at asingle point, such as shown by 606 a in FIG. 42, the engagement of themilling cutter when following the tool path gradually increases to thepredetermined value of engagement and then gradually decreases. Thiscondition satisfies the following equation:Re=2Rs−(Ra+Rt),   (21)

where Rs is the radius of the previous material boundary 606 a and Ra+Rtis the radius of the enlarged material boundary 606 b.

A value for the radius of the tool path for milling an enlarged materialboundary 606 b, where the maximum engagement of the milling cutterequals the predetermined maximum engagement, is found by substitutingequation 21 into equation 20 resulting in:

$\begin{matrix}{{Rao} = {2{{Rs}\left( \frac{{Rs} - {Rt}}{{2{Rs}} - {Rt} + {{Rt}\;{\cos E}}} \right)}}} & (22)\end{matrix}$

If Ra<Rao, the engagement circle does not intersect the previousmaterial boundary and the engagement will be less than the predeterminedmaximum engagement. If Ra>Rao, the engagement circle 610 intersects theprevious material boundary 606 a at two points. In this case the maximumengagement of the milling cutter will exceed the predetermined maximumengagement over a range of points along the enlarged material boundary606 b. Insertion of a transition portion in the previous materialboundary according to the ninth preferred embodiment may be used tomitigate the excess engagement.

It would be clear to one skilled in the art that equation 22 can be usedas a basis for increasing the size of a material boundary without limit,all the while maintaining the engagement of the milling cutter to beless than a predetermined value, by simply constructing a succession ofincreasing radius passes and substituting the value of the radius of theprevious pass Ra for the value of Rs. Further, the method is not limitedto the circular pocket shown in FIG. 41. The method may also be used toincrease the size of a circular arc, such as might be desired to bemilled in a part boundary. Also, the method may be used to enlarge anyshape, provided the starting boundary is substantially circular.

It may be seen that the present invention describes a method fordirectly generating a tool path which is based on controlling theengagement of the milling tool with the workpiece. Each tool pathencompasses a substantial portion of a defined region. The disclosedembodiments of the invention are adapted to generating tool paths forregions of differing geometry, but all have the common characteristicthat maintaining the relative constancy of the engagement of the millingcutter is decisive in determining the tool path. The result ofpreferentially controlling the engagement of the milling cutter is a CNCprogram which removes material in a shorter time than CNC programsgenerated by existing CAM programs while providing improved tool life.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A method for generating a spiral-like tool path for milling a regionof a workpiece, the method comprising the steps of: creating a family ofconcentric indexed circular arcs at each of two or more separate anddistinct selected points within the region; determining, using acomputer, parameters of a first set of blends to connect together thecircular arcs of adjacent families of the circular arcs having anidentical index to form a plurality of isoloops; determining, using acomputer, parameters of a second set of blends for blending betweenadjacent isoloops to form the spiral-like tool path, and generatinginstructions for controlling a milling cutter to mill the region inaccordance with the generated spiral-like tool path.
 2. The method ofclaim 1, wherein a radius of each concentric arc, the parameters of eachfirst set of blends and the parameters of each second set of blends areselected such that an engagement of the milling cutter following thetool path does not exceed a predetermined value of cutter engagement. 3.The method of claim 1, wherein a radius of each concentric arc, theparameters of each first set of blends and the parameters of each secondset of blends are selected such that an engagement of the milling cutterfollowing the tool path substantially equals a predetermined value ofcutter engagement.
 4. The method of claim 1, wherein the selected pointslay on a medial axis of the region.
 5. The method of claim 4, whereinthe locations of branch points on the medial axis are determined usingthe medial axis transform.
 6. The method of claim 1, wherein a largestcircular arc in each family of concentric circular arcs is tangent tothree sides of the region.
 7. A non-transitory computer readable storagemedium for having an executable program for generating a spiral-liketool path for milling a region of a workpiece stored thereon, whereinthe program instructs a computer to perform the steps of: creating afamily of concentric indexed circular arcs at each of two or moreseparate and distinct selected points within the region; determiningparameters of a first set of blends to connect together the circulararcs of adjacent families of the circular arcs having an identical indexto form a plurality of isoloops; determining, parameters of a second setof blends for blending between adjacent isoloops to form the spiral-liketool path, and generating instructions for controlling a milling cutterto mill the region in accordance with the generated spiral-like toolpath.
 8. The non-transitory computer readable storage medium of claim 7,wherein a radius of each concentric arc, the parameters of each firstset of blends and the parameters of each second set of blends areselected such that an engagement of the milling cutter following thetool path does not exceed a predetermined value of cutter engagement. 9.The non-transitory computer readable storage medium of claim 7, whereina radius of each concentric arc, the parameters of each first set ofblends and the parameters of each second set of blends are selected suchthat an engagement of the milling cutter following the tool pathsubstantially equals a predetermined value of cutter engagement.
 10. Thenon-transitory computer readable storage medium of claim 7, wherein theselected points lay on a medial axis of the region.
 11. Thenon-transitory computer readable storage medium of claim 10, wherein thelocations of branch points on the medial axis are determined using themedial axis transform.
 12. The non-transitory computer readable storagemedium of claim 7, wherein a largest circular arc in each family ofconcentric circular arcs is tangent to three sides of the region.
 13. Amachine for generating a spiral-like tool path for milling a region of aworkpiece comprising: a computer coupled to a memory, wherein thecomputer is programmed to generate the tool path by the steps of:creating a family of concentric indexed circular arcs at each of two ormore separate and distinct selected points within the region;determining parameters of a first set of blends to connect together thecircular arcs of adjacent families of the circular arcs having anidentical index to form a plurality of isoloops; determining, parametersof a second set of blends for blending between adjacent isoloops to formthe spiral-like tool path, and generating instructions for controlling amilling cutter to mill the region in accordance with the generatedspiral-like tool path.
 14. The machine of claim 13, wherein a radius ofeach concentric arc, the parameters of each first set of blends and theparameters of each second set of blends are selected such that anengagement of the milling cutter following the tool path does not exceeda predetermined value of cutter engagement.
 15. The machine of claim 13,wherein a radius of each concentric arc, the parameters of each firstset of blends and the parameters of each second set of blends areselected such that an engagement of the milling cutter following thetool path substantially equals a predetermined value of cutterengagement.
 16. The machine of claim 13, wherein the selected points layon a medial axis of the region.
 17. The machine of claim 16, wherein thelocations of branch points on the medial axis are determined using themedial axis transform.
 18. The machine of claim 13, wherein a largestcircular arc in each family of concentric circular arcs is tangent tothree sides of the region.